In a motor vehicle, this vehicle electrical system serves to supply the various pieces of electrical equipment with which the vehicle is fitted. The supply is provided by at least one battery. The latter is charged, by means of a rotary electrical machine, from the energy produced by the rotation of the heat engine of the vehicle. By rotary electrical machine, what is meant, in the widest possible sense, is any single-phase or multi-phase rotary electrical machine which is used to produce d.c. output current supplying the vehicle electrical system. It may in particular be an alternator or an alternator-starter.
In the event of an electrical load suddenly being disconnected from the vehicle electrical system, or from a battery, or from both, a phenomenon known as a load dump occurs which causes an over-voltage in the vehicle electrical system. What happens is that, because a system for regulating a generating current in the machine is unable to act quickly enough following a load dump, the machine continues to produce the same output current even though the current consumption on the side on which the vehicle electrical system is situated has dropped.
Conventionally, the battery of the vehicle is a 14 volt battery. In principle, due to its low internal resistance, it limits the voltage peaks which occur in the vehicle electrical system of the vehicle in the event of a load dump to about 17 volts. This battery thereby absorbs the small over-voltages. Nevertheless, in the event of the battery being disconnected (due for example to a supply cable breaking), a very high over-voltage may occur in the vehicle electrical system. What happens is that the current produced by the machine charges the capacitances (including the parasitic capacitances) connected to the vehicle electrical system, and consequently causes a large rise in the d.c. voltage on the vehicle electrical system.
There is a risk of this over-voltage damaging the pieces of electrical equipment supplied by the vehicle electrical system. This is why all the vehicle's pieces of electrical equipment are sized to withstand a maximum voltage of about 32 volts, which corresponds to an over-voltage of about 20 volts.
There are various known solutions which enable the voltage on the vehicle electrical system to be limited to a maximum permitted voltage, i.e. the highest voltage which the pieces of electrical equipment of the vehicle are able to withstand without the risk of their being damaged.
The solution which is explained in patent application FR 2874765 takes account of the future use of 42 volt batteries in vehicles, as is made necessary by the tendency of pieces of electrical equipment with a high energy consumption to increase in number. The fact is that, under the specifications laid down in a future European standard (still being written), the pieces of electrical equipment connected to the vehicle's electrical system will probably have to be sized to operate at a voltage of up to 48 volts, and to withstand a maximum voltage of 58 volts, which corresponds to an over-voltage of only 10 volts.
In a conventional basic architecture where a control and power module for a rotary electrical machine having a plurality of phase windings comprises:                a power circuit comprising which comprises a plurality of arms which are intended to be associated with a plurality of phase windings, and        a control circuit which is constructed to operate the power circuit when the machine is operating in a nominal mode,        
the solution which is described in document FR 2874765 proposes the construction of the control circuit in order, in addition:                to monitor an output voltage from the power circuit,        to lock at least one arm of the power circuit in a conductive state when the output voltages reaches a first threshold level, such that the machine operates in a degraded mode,        to return the machine to operation in the nominal mode when the output voltage reaches a second threshold level.        
However, this criterion for returning to the nominal mode of operation, based as it is on the output voltage, has the disadvantage of not being representative of the actual change in state, magnetically speaking, of the rotary machine, which is the only thing which enables the load dump on the electrical system to be compensated for effectively.